So yesterday I had to jump through some hoops getting Xcode set up, in the end I had to install it once, uninstall it and then install a different version.

At the start I had ~78GB free. After the first install it went down to ~75GB as expected. However when I uninstalled it and installed the other version which requires 10+ GB since it includes the iOS SDK (the version I uninstalled was what came on the disk with my machine). Now my disk usage reads at ~62 GB, not quite I expect considering my starting amount and the download (4 GB) and the install (10 GB). My question: is there some cache that just hasn't been cleared yet or is finder just not reporting the disk usage correctly?

Is this after a reboot (safe mode to get a fsck)? There are reports the app store is occasionally leaving downloads in a partially deleted state. You will find lost+found directories if this is the case.
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bmike♦Aug 2 '11 at 21:37

I didn't install from the app store (I don't want to buy lion just yet), first install was from disc that came with my machine and the second was from the Apple Dev site
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primehunter326Aug 2 '11 at 22:06

I would run the /Developer/Library/uninstall-devtools --help and clean up followed by a reboot. If the space is still missing, try the fsck or safe boot, then dig deeper. I've never heard of this acting badly on Xcode 3 - but it still could be something related to the install.
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bmike♦Aug 2 '11 at 22:16

I did uninstall-devtools --mode=all to remove it the first time and that seemed pretty thorough (Developer folder was completely removed). I'm a little wary of adding the --help flag because adding --? (which should have the same effect) caused it to delete stuff and completely ignore the flag. I'll try fsck and see if that does anything. On an semi related note, does the version of gcc bundled with Xcode always have some really long name (when I invoke it in terminal it appears as i686-apple-darwin10-gcc-4.2.1
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primehunter326Aug 3 '11 at 4:08

I know that Finder is not including the space for MobileBackups as used space, but it seems to be connected to backups anyway. I observed that in the "Storage" Tab of "About this Mac" the "Other" space fluctuates heavily, in particular its size grows when you use space, and does not immediately go down when you free space, there seems to be a delay.

I observe the same and for a while I thought it has to do with MobileBackups, but these effects remain after disabling MobileBackups. What Finder (or the Storage tab) displays seems to fluctuate with little connection to reality. I have the experience that the "lost space" will return after a week or so, indicating that it has to do with some sort of backups.